The present invention concerns electric machines, especially but not exclusively small motors, whose field is established by permanent magnets, in particular at least one pair of diametrically located field magnets which define a radial air gap with the armature of the machine and each having the form of a generally arcuate permanent-magnet segment, as so called here. Typically, the radial thickness of each permanent-magnet segment decreases in the circumferential direction of the motor in either direction towards the ends of the magnet segment, the radial thickness of the air gap increasing in correspondence thereto.
In this type of permanent-magnet-field electric machine or motor, the progressive increase of the radial length of the machine's air gap proceeding in the direction towards either end of each permanent-magnet segment serves to suppress the magnetic noise of the machine or motor. Especially when use is made of permanent magnets having high magnetic values and accordingly high energy densities, the level of force to which the successive teeth of the motor's rotating armature are subjected is greatly increased on account of the increased magnetic flux passing through the armature teeth. The increased energy density of the permanent magnets produces increased air-gap induction which results in an enormous discontinuity in the flux passing through the armature teeth as they successively move into the working field of the permanent magnets; this, in turn, results in a very considerable increase in that noise whose fundamental frequency corresponds to the rate at which successive armature teeth pass the permanent-magnet segments. Indeed, it is to lower this magnetic noise down to a more normal level that use can in particular be made of the technique mentioned above, namely using an air gap whose radial length increases progressively towards either end of each permanent-magnet segment, inasmuch as this serves to reduce the flux passing through the air gap in the region of the two ends of each such magnet segment.
However, it is to be noted that the magnetic actions which tend to demagnetize the permanent-magnet segments to sucha machine or motor, attributable to the quadrature field of the machine's armature, are of greater and greater potentially demagnetizing effect as one proceeds towards the trailing end of each permanent-magnet segment, i.e., the end of the segment last reached by an arbitrarily selected point on the rotating armature. Furthermore, the ability of such permanent-magnet segment to withstand demagnetizing forces decreases with decreasing radial thickness of the magnet segment. As a result, if the radial thickness of the magnet segment is made progressively smaller towards its ends in order to suppress magnetic noise in the manner set forth above, this has the simultaneous and very undesirable effect of increasing the magnet segments' susceptibility to demagnetization.
It is already known to attempt to provide permanent-magnet segments of greater resistance to demagnetization, by forming such magnet segment from two subsegments, each made of a magnetic material of different respective magnetic characteristics, in particular one material being of higher remanence and the other having a higher coercive force. The danger of demagnetization, which arises most especially during start-up at low operating temperatures, exists most markedly at the trailing ends of the permanent-magnet segments because, as already indicated, it is there that the armature quadrature field is strongest. For this reason, the permanent-magnet subsegment of higher coercive force but lower magnetic remanence has conventionally been made the trailing one of the subsegments of the permanent-magnet segment. Then, in order to optimize the utilization of the total volume available for each such permanent-magnet segment, the ratio of the volume of the subsegment of higher coercive force to the total volume of the magnet segment has been made equal to the ratio of the coercive force of the higher-remanence material to the coercive force of the lower-remanence material. Such permanent magnets are usually denoted two-component magnets.
If one attempts to use two-component permanent magnets as the permanent-magnet segments of such machine, i.e., in order to compensate for the increased susceptibility to demagnetization, this does not actually lead to an overall improvement. Two-component permanent-magnet segments have been found to become demagnetized at the abutment joints between their two component subsegments.